High-energy solid formulations, such as propellants, explosives and gasifiers, generally consist of particulate solids, such as fuel material, oxidizers or both, held together by an elastomeric binder. These formulations may also include a liquid plasticizer, such as a nitrate ester, which contributes to the elastomeric characteristics of the binder and adds additional energy to the formulation.
While the elastomeric binder matrix is an important means of dispersing and immobilizing the fuel material and oxidizer, the materials used in the binder burn with substantially lower energy than does the fuel material. The binder thus imposes a limit on the energy content available from the fuel material. One way to minimize this limitation is to use an elastomeric binder which releases as much energy as possible when burning with the fuel material. It is desirable, therefore, that the elastomeric binder have pendant groups which themselves are relatively high in energy.
To this end, 3,3-bis(azidomethyl)oxetane (BAMO) and 3-azidomethyl-3-R.sup.1 -oxetanes (wherein R.sup.1 is, for example, hydrogen, lower alkyl, alkoxy, hydroxy, NF.sub.2, ONO.sub.2, NO.sub.2, etc.) are useful monomers since polyethers prepared from these oxetane compounds can be subsequently cured to form high-energy binder materials. It has been determined that in addition to retaining the necessary characteristics of a binder, such as good elastomeric and mechanical strength properties, polyethers containing mono- and bis(azidomethyl)oxetanes are sufficiently high in energy and sufficiently miscible with nitrate ester plasticizers to be useful as elastomeric binders in propellant formulations and other energetic compositions.
Unfortunately, the use of the energetic azido pendant group in oxetane monomers has been severely limited by economic, environmental and safety considerations. Currently, the reaction of inorganic azides with organic halides to give organic azides, e.g., BAMO, is carried out in dipolar, aprotic solvents such as dimethylformamide (DMF) or dimethylsulfoxide (DMSO). Solutions of sodium azide in DMF and DMSO are, however, hazardous and, thus, are unsuitable for industrial scale reactions. Sodium azide is highly toxic, but when in DMSO or DMF, it is even more hazardous. DMSO and DMF will carry sodium azide into the blood stream and, thus, great care must be taken to avoid contact with such sodium azide solutions. Moreover, sodium azide is not real soluble in DMSO or DMF and, thus, it must be slowly added to such organic solvents while stirring. Unfortunately, sodium azide is very impact sensitive and if, for example, the stirring paddle contacts the solid sodium azide, an explosion can result. In addition, it is extremely difficult to control the pH of such solutions so as to prevent the formation of hydrazoic acid which is highly explosive and toxic. Further, when either DMF or DMSO is used as the solvent, an excess of sodium azide must be used because under the reaction conditions employed, sodium azide gets destroyed. Unfortunately, when energetic materials are used, it is hazardous to open up the reaction system to add additional amounts of sodium azide. Moreover, at the end of the reaction, there is an excess (i.e., about a 10% to 20% excess) of sodium azide which must now be removed. Unfortunately, the removal of excess sodium azide and, in turn, the isolation of the final product from the organic reaction mixture involve multiple steps.
In view of the foregoing, there exists a need for a process for the preparation of 3,3-bis(azidomethyl)oxetane (BAMO) as well as other 3-azidomethyl-3-R.sup.1 -oxetanes (wherein R.sup.1 is, for example, hydrogen, lower alkyl, alkoxy, hydroxy, NF.sub.2, ONO.sub.2, NO.sub.2, etc. ) that overcomes the economic, environmental and safety limitations of the prior art methods. Quite surprisingly, the present invention remedies this need by providing such a process.